1. Field of the Invention
The present invention generally relates to a traction control apparatus, and more particularly to a traction control apparatus for an automotive vehicle equipped with an automatic transmission, in which an improved traction control process is performed with an appropriate return torque when a shifting of the automatic transmission occurs.
2. Description of the Related Art
A traction control system for use in automotive vehicles with automatic transmissions is known. In an automotive vehicle equipped with an automatic transmission, a traction control process is performed by the traction control system to prevent the driving wheels of the vehicle from undergoing excessive slippage when the vehicle starts moving or when it is accelerated. By performing the traction control by means of the traction control system, vehicle acceleration and straight line stability are improved when the vehicle is in such operating conditions.
In the above mentioned traction control system, a target rotational speed is predetermined for each of the driving wheels. A slipping amount is defined to be a speed difference between the predetermined target rotational speed and the actual speed of the driving wheels of the vehicle. In the traction control system, a feedback control process for the driving torque is performed so as to maintain the slipping amount mentioned above at a value smaller than a predetermined value. In this specification, the feedback control process for maintaining to maintain the above mentioned slipping amount at a value smaller than a predetermined value is hereinafter called the traction control process.
In the above mentioned traction control system, if a shift-up action of the automatic transmission occurs when the traction control process is being performed, the throttle valve is closed to cause the position of the throttle valve to change to a smaller opening position. Thus, the engine speed is lowered so as to prevent an overload of the engine at this time. If no measure is taken at this time, the engine speed is increased excessively since no driving torque from the engine is transmitted to the driving wheels via the automatic transmission during the shift-up action. When the shift-up action of the automatic transmission ends, the engine speed is increased and the traction control process is returned to the normal condition thereof.
Japanese Laid-Open Patent Publication No. 1-247726 discloses an automotive vehicle traction control method of the type described above. In the traction control method disclosed in this publication, when a shift-up action of the automatic transmission occurs during a traction control process, the throttle valve is closed to lower the engine speed. After the shift-up action is completed, the throttle valve is opened to increase the engine speed so that the traction control process is returned to the normal condition thereof. A return position of the throttle valve by which the traction control process is returned to the normal operation thereof is determined based on the position of the throttle valve before the shift-up action occurs and based on the gear position of the automatic transmission after the shift-up action occurs.
However, the position of the throttle valve before the shift-up action occurs may be an inappropriate position due to external conditions such as: a fuel cut condition, an ignition timing retarded condition, or a local riding transfer slip. In the above mentioned traction control method, there is a problem in that the return position of the throttle valve may be determined based on an inappropriate position of the throttle valve before the shift-up action occurs. If an inappropriate position of the throttle valve is used it is impossible to set the return position of the throttle valve to the position at which an optimal traction control process suitable for the current road conditions and the engine operating conditions is performed.